1. Technical Field
The present invention relates to a variable planar light guide module (VPLG). More particularly, the invention relates to a VPLG designed for motor vehicles and intended to be placed on both sides of the vehicle.
2. Prior Art
A light guide or planar light guide works on the principle of total internal reflection. Current light guides are usually placed in the lamp assembly so that most of the light guide's decoupling surface is perpendicular to the intended direction of the decoupled light rays, which is parallel to the longitudinal axis of the vehicle. When incorporated into an automobile, lamps of this type become very attractive and create a unique appearance.
Light guide techniques are based on the total internal reflection (TIR) principle, which is caused by two materials with different refractive indices.
According to known technology, as seen in FIG. 1, coupled light 201 from a light source 200 goes through a translucent light guide 202 by TIR, which occurs on the smooth inner wall 204 of the light guide 202. To decouple the light from the light guide 202, prismatic structures 206 are placed on one side of the light guide 202. The prismatic structures 206 change the direction of impacted rays. The prismatic structure 206 defines the direction of decoupled light and must be designed so that, for impacted rays, the condition of TIR is fulfilled. Upon contact with the prismatic structures 206, the direction of these impacted rays is changed and the rays are reflected to the output wall 208 of the light guide 202, where the condition for TIR is not fulfilled. Thus, the rays 201 leave the light guide 202 through the output wall 208 as decoupled light rays 210.
The described optical system works with a good efficiency only in the situation where the output wall 208 of the light guide 202 is perpendicular to the decoupled light 210. In use with an automotive vehicle, the main direction of decoupled light is given by the lighting function represented by the light guide. Therefore, the output wall 208 of the light guide 202 must typically be perpendicular to the direction of the longitudinal axis of the vehicle. The efficiency of the decoupled light being emitted in this desired direction rapidly drops with the changes in the angle β from perpendicular, between the output wall 208 of the light guide 202 and longitudinal axis X of the vehicle. The reason for this drop in efficiency is that impact of the coupled light rays with the prismatic structure does not occur via TIR reflection, but rather with Fresnel reflection, and therefore has a very low efficiency of reflection.
In the case of TIR reflection, 100% of the light is reflected to the output wall of the light guide. In the case of Fresnel reflection, only approximately 5%-20% of the light is reflected to the output wall. The shape of the prismatic structure gives the precise percentage of Fresnel reflection. FIG. 2 illustrates the situation where the light guide 202 is not perpendicular to the longitudinal axis X of the vehicle. FIG. 3 is a graph showing the drop in efficiency relative to a changing β angle from perpendicular.
Using current planar light guide technology means that the efficiency of whole optical system will be very low and not able to fulfill legal requirements for a rear position light of an automotive vehicle because such rear lamps are not typically oriented perpendicular to the longitudinal axis of the vehicle.
It is therefore an object of the present invention to provide improved optical efficiency of the decoupled light from a light guide allowing inclined lamps to send light in a requested direction, generally parallel to the longitudinal axis of the vehicle.